This invention relates to an elastic surface wave device using an X-cut LiTaO.sub.3 substrate as a piezoelectric substrate.
It would be contemplated to apply an elastic surface wave device to a picture intermediate frequency (PIF) filter circuit of a television receiver. In this case, a ceramic or LiNbO.sub.3 substrate is normally examined for a piezoelectric substrate of the elastic surface wave device. The ceramic substrate, however, is poor in the manufacturing yield and unsuitable to an industrial application, whereas the LiNbO.sub.3 substrate manifests a poor temperature characteristic and it is unsuited as the PIF filter of the television receiver.
The inventors have developed the use of a LiTaO.sub.3 substrate. The LiTaO.sub.3 substrate is high in cost and, coupled with no available proper cutting method, it has not been put to practice. The inventors have discovered that the temperature characteristic and bulk spurious characteristic are prominently improved by setting the direction of propagation of an elastic surface wave at an angle of 67.8 to 142.degree. to the Y-axis of the LiTaO.sub.3 crystal. If in this case the substrate has no proper configuration, the spurious characteristic is not yet improved and there is a consequent poor yield. This proves a bar to the realization of such devices.
By way of example, a PIF filter for a TV set was manufactured using as a piezoelectric substrate an X-cut LiTaO.sub.3 substrate whose surface wave is propagated in a 112.degree. direction to the Y-axis of the substrate. The frequency (MHz)-relative response output characteristic was measured, the result of which is as shown in FIG. 1. The curve A of FIG. 1 indicates the frequency characteristic of the PIF filter band when the relative response output (dB) is plotted in units of 10dB. From FIG. 1 it appears that no apparent great ripple emerges, but when the relative response output is plotted in units of 1dB for the 10-fold sensitivity, a relatively great ripple is observed, as indicated by the curve B, over 56 to 57 Hz of the corresponding filter pass band.
For a filter of the frequency characteristic of FIG. 1 the reflection characteristic measured by using a network analyzer is as shown in FIG. 2. From FIG. 2 it will be seen that, in addition to the excitation of a fundamental wave and 2nd order harmonics of the surface wave, a strong resonance occurs at a plurality of frequency positions such as 6.108, 18.823, 31.376, 43.959 and 56.489 MHz. When such a resonance occurs in the PIF filter pass band, a surface wave excitation energy is absorbed by the resonance, with the resultant possible ripples in the PIF filter pass band. When the resonant frequency agrees with a trap frequency requiring a greater attenuation, no requisite attenuation will be obtained.
It is accordingly the object of this invention to provide an elastic surface wave device which, based on the consideration of FIGS. 1 and 2, can beforehand prevent the absorption of bulk spurious response excitation energies due to resonance to reduce ripples i.e. spurious components.